Solid metal tips (SMTs) are typically integrated into wind turbine blades to address the problem of lightning strikes. The SMT acts as a lightning receptor that receives lightning and discharges it to a ground potential via conductors that extend inside the blade, nacelle, and tower of the wind turbine. SMTs therefore allow lightning to be discharged safely and minimise the risk of damage to the wind turbine from lightning strikes.
To put the invention into context and to explain certain problems suffered by the prior art, reference will firstly be made, by way of example, to FIGS. 1 to 3 of the accompanying drawings, in which:
FIG. 1 is a perspective view of a wind turbine blade comprising an SMT;
FIG. 2 is a partial perspective view of a tip region of a mould half that forms part of a mould assembly for making the wind turbine blade of FIG. 1;
FIGS. 3A and 3B illustrate a known method of making a wind turbine blade using the mould half of FIG. 2; and
FIG. 3C illustrates the effect of an underbite misalignment of a mould assembly used in the method of FIGS. 3A and 3B.
FIG. 1 is a perspective view of a typical wind turbine rotor blade 10 that includes an SMT 12 which acts as a lightning receptor. In addition to the SMT 12, the blade 10 comprises an outer shell 14 that is fabricated from two half shells: a first half shell 16 and a second half shell 18. The half shells 16, 18 are laminated structures that are moulded from glass-fibre reinforced plastic (GRP). A lightning down conductor cable 13 is disposed within the blade 10 and is connected to the SMT 12 and carries lightning current from the tip of the blade to the root of the blade and on to ground potential The half shells 16, 18 are typically moulded in separate mould halves, such as the mould half 20 that is shown in part in FIG. 2.
To form a half shell 16, 18, its component layers are first laid up in the mould half 20. An outer skin in the form of a dry glass-fibre cloth is placed on a mould surface 22 of the mould half 20. Structural layers are then laid up on the outer skin, and an inner skin in the form of a dry glass-fibre cloth is then placed over the structural layers. The half shells 16, 18 are laid up so as to extend to a distance short of the tip 24 of the mould half 20, such that the tip region of the half shell 16, 18 is truncated to provide space for the SMT 12.
Next, the components of the half shell 16, 18 are covered with an airtight bag to form an evacuation chamber encapsulating all of the components. The chamber is evacuated, a supply of liquid resin is connected to the chamber, and resin is introduced into the mould half 20. The resin infuses between the components, and the assembly then undergoes a curing cycle to harden the resin. It will be appreciated that pre-preg composite materials may be used instead of dry materials, thereby avoiding the resin infusion step.
Referring to FIG. 3A, once each half shell 16, 18 has been moulded, the SMT 12 is arranged in place at the tip 24 of one of the two mould halves 20, and is integrated into the associated half shell 16, for example by means of an adhesive. Finally, the two half shells 16, 18 are brought together by closing the mould, as shown in FIG. 3B, and the half shells 16, 18 are bonded together to form the complete blade 10.
Although such a method provides a simple way of integrating an SMT 12 with a wind turbine blade 10, problems can arise if there is any misalignment between the two mould halves 20 when the mould is closed. If the mould halves 20 are not aligned correctly, as shown in FIG. 3C, the upper mould half 20a and the upper half shell 18 will not be positioned correctly with respect to the SMT.
For example, if the upper mould half 20a lies rearward of the lower mould half 20b, the mould exhibits a so-called ‘underbite’, as shown in FIG. 3C. In this event, the upper mould half 20a may clash with the SMT 12 when the mould is closed. In extreme cases, this can damage the SMT 12, or even lead to the SMT 12 breaking away from the lower half shell 16.
Even if the SMT 12 is not damaged, the clashing can prevent the upper mould half 20a from being lowered in to position correctly, leaving an unacceptable gap 26 between the upper and lower half shells 16, 18 and a further gap 28 between the upper half shell 18 and the SMT 12. This is particularly problematic, as the gap 26 between the upper and lower half shells 16, 18 leads to insufficient contact occurring between the half shells 16, 18, which may ultimately compromise the bond strength between the shells 16, 18 and the structural integrity of the blade 10.
Such clashing can also cause damage to the mould itself. For example, the smooth inner surface of the mould may be damaged, which can be problematic for future use of the mould.
A misalignment of only a few millimeters is sufficient to cause the clashing described above. Modern wind turbine blades may be over eighty meters in length, and it will be appreciated that when manufacturing blades of this size, misalignments are difficult to avoid. In particular, it is difficult to achieve precise alignment simultaneously at both the blade root and at the blade tip. Thus, for manufacturing processes where alignment at the root is crucial, alignment of the half shells at the tip is particularly difficult, and the above problem is particularly prevalent.
It is an object of the invention to mitigate or overcome this problem.